Tanned skin is a commonly desired aesthetic element of personal appearance. A tanned appearance is associated with looking healthy, youthful and generally more attractive.
The natural tanning process is triggered by exposure to sunlight, particularly, ultraviolet (UV) light, and occurs when the skin expresses the pigment melanin. Melanin serves a photoprotective function arising from its photochemical properties. It absorbs harmful UV rays and transforms the energy into heat. This prevents the indirect DNA damage which is associated with the formation of malignant melanoma and other skin cancers.
The ultraviolet frequencies responsible for tanning are often divided into the UVA and UVB ranges. UVA causes the release of preexisting melanin from the melanocytes to combine with oxygen (oxidize), which in turn creates the actual tan color in the skin. UVB induces increased melanin production. UVB exposure will result in a tan about 2 days after exposure. UVA, and particularly UVB, can cause the formation of reactive oxygen species which in turn damage DNA both directly and through various biochemical pathways. The harmful effects of UV radiation are well understood, and UV radiation has been linked to more cancers worldwide than any other carcinogen. Nyugen et al., Nonmelanoma skin cancer, Curr Treat Opt Oncol 3:193-203 (2002).
Tanning beds have been used to mimic the effects of sunlight. Tanning beds use several fluorescent lamps that have phosphor blends designed to emit UV in a spectrum that is effective to cause tanning. Most tanning beds emit mainly UVA radiation.
To avoid the risk of UV radiation associated with sunlight or tanning beds, products have been developed to provide a suntanned look without the UV radiation. Thus, sunless tanning products, including self-tanners and spray tans have been developed to provide the appearance of a tan.
Most of the sunless tanning products available are lotions and sprays that contain dihydroxyacetone (DHA) as the active ingredient. DHA reacts chemically with amino acid groups which are part of the dead layer of keratin on the skin surface. Various amino acids react differently to DHA, producing different tones of coloration from yellow to brown. The resulting pigments are called melanoidins. These are similar in coloration to melanin. The artificial tanning coloring resulting from use of DHA based products can vary in color and evenness depending on the characteristics of the user's keratin layer. In some artificial tanning products erythulose is used in addition to DHA. Erythulose works in the same way as DHA on the skin surface, but provides a different color range. Juglone, and lawsone have also been used as bronzing agents.
Other agents used to produce a sunless tan include skin surface coloring agents, such as synthetic dyes, natural colorants and iron oxides. These cosmetic approaches do not provide a satisfactory, natural-appearing, long-lasting tan. In addition, colorants can wash off or cause staining of fabrics,
There are two principal types of melanin associated with skin coloration: eumelanin and pheomelanin. Eumelanin provides black or brown skin coloration, while pheomelanin provides pale pink skin and red hair coloration.
Melanin is produced from tyrosine by the enzyme tyrosinase (formed by melanocyte skin cells). Tyrosinase converts tyrosine into dihydroxyphenylalanine (I-dopa), and then into dopaquinone. Dopaquinone is then converted into eumelanin or phaeomelanin.
Products have been proposed which stimulate the production of melanin in the skin. For example, the use of melanin precursors such as tyrosine and the enzyme tyrosinase, which is required to catalyze the formation of melanin, have been proposed For example, U.S. Pat. No. 4,515,773 (Herlihy) is directed to compositions for tanning the human epidermis. The compositions contain a 10 mM to 1 M concentration of precursors such as tyrosine, phenylalanine and catechol derivatives distributed throughout a base along with the enzyme tyrosinase.
U.S. Pat. No. 5,061,480 (Marchese et al.), directed to skin tanning compositions, discloses compositions with tyrosine active ingredient working in synergy with a non-ionic surfactant (e.g., polyoxyethylene ethers and polyoxyethylene sorbitan fatty acid esters such as BRIJ, TWEEN or ARLACEL) to increase the rate of penetration into the skin. Addition of riboflavin or adenosine triphosphate is disclosed to accelerate the oxidation process which produces skin tanning pigment. The tyrosine or derivative of tyrosine is taught to be present at 0.2 to 0.5 weight percent.
However, such products have not been effective in providing meaningful sunless melanin production and have not been commercially successful.
U.S. Patent Publication 2005/0175556 (Gupta), the disclosure of which is hereby incorporated by reference, is directed to skin darkening compositions based on enhancement of melanin synthesis by tyrosinase enzyme. Gupta discloses topical skin darkening compositions comprising (i) at least one tyrosine substrate agent, (ii) at least one tyrosine activator agent, and optionally, a pharmaceutical carrier. All examples teach Mucuna prurience extract or dihydroxy phenylalanine (D- and L-dopa) as the tyrosinase substrates. Copper ATP and copper glutathione are taught as tyrosine activator agents.
However, there remains a need in the industry for sunless tanning compositions that are effective and do not require activation using synthetic compounds or metals. Desirably, sunless tanning compounds will be formulated using naturally-occurring plant-based compounds as the tyrosine activators.